1. Field of the Invention
The present invention relates to a digital modulation method which converts 8-bit digital data into 14-bit digital modulation codes.
2. Description of the Prior Art
Conventional apparatuses, which use rotary heads to record digital data to magnetic tape or to reproduce digital data recorded on magnetic tape, utilize rotary transformers to record or reproduce the digital data: recording is performed by supplying the digital data to the rotary head through the rotary transformer; and reproduction is performed by reading the digital signal with the rotary magnetic head through the rotary transformer.
Consequently, if the reproduced signal includes a DC (Direct Current) component, the digital data cannot be correctly reproduced. For this reason, the digital data must be recorded by using a DC free digital modulation system.
Among the conventional DC free digital modulation systems, the following systems are well known.
The 8-10 modulation system, the DR (Density Ratio) of which is 0.8, is described in Japanese Patent Application Laying-Open No. 56-19506.
The M.sup.2 modulation system, the DR of which is 1, is known.
The 8-14 modulation system, the DR of which is 1.14, is described in Japanese Patent Application Laying-Open No. 61-196469. This system provides up to four 14-bit digital modulation codes for each 8-bit digital data. When the CDS (Code word Digital Sum) of a 14-bit modulation code is zero, the code is paired with the reversal pattern thereof. When the CDS of a 14-bit digital modulation code is not zero, the code is grouped with the following three codes: another 14-bit modulation code the absolute value and sign of CDS of which differ from those of the above code; and the reversal patterns of the respective codes.
Here, CDS is defined as a DSV calculated from the first bit to the last bit of a modulation code: DSV (Digital Sum Value) is a total sum obtained by adding -1 for respective bits "0" in a series of digital modulation codes and by adding 1 for respective bits "1" in the same codes. The reversal pattern is a pattern obtained by reversing each bit in a code: bit "1" is reversed to "0", whereas bit "0" is reversed to "1".
The above-mentioned conventional modulation systems have the following problems.
The 8-10 modulation system is not appropriate to a high-density recording because of its low DR of 0.8.
The M.sup.2 modulation system is restricted in its high-density recording because of its DR of 1.
The 8-14 modulation system has up to 4 modulation codes for each 8-bit code, and the absolute value of CDS of the digital modulation codes are allowed up to 6. In addition, DSV at the end of each 14-bit digital modulation code in the code stream is allowed up to .+-.4, and DSV at each bit in a series of the 14-bit digital modulation codes is allowed up to .+-.9. Consequently, it is difficult to eliminate the DC component of the modulation codes in a short time, and hence, low frequency component must be adequately passed in a recording/reproducing system including the rotary transformer.
A further problem is presented in the 8-14 modulation system. Generally speaking, magnetizing depth on magnetic tape is about 1/4 of the magnetized wavelength. When recording signals are over-written on the tape, the following problem occurs: recording a new signal of the shortest magnetized wavelength on the longest magnetized wavelength which is 4 times or more longer than the shortest magnetized wavelength results in the erasing residue in the deeper part of the recording medium. This erasing residue appears during reproduction of the new signal, and so the over-writing is practically difficult.
Thus, the 8-14 modulation system suffers from the problem caused by the erasing residue when over-writing is performed because the number of consecutive identical bits ("0" or "1") in a 8-14 modulation code train is 2-9.
Incidentally, in the later description, the term "consecutive identical bits" means two or more consecutive identical bits: for example, "000" or "11".